Crack Path Selection in Orientationally Ordered Composites

While cracks in isotropic homogeneous materials propagate straight, perpendicularly to the tensile axis [1], they often deflect from a straight path in natural [2, 3] and man made [4] composites for reasons that are not well understood. Here we combine experiment and theory to identify key emergent properties that predict whether cracks propagate straight or kink on a macroscale larger than the composite microstructure. Those properties include the anisotropy of the fracture energy, which we measure and control by varying the volume fraction of orientationally ordered alumina platelets inside a polymer matrix, and a microstructure-dependent process zone size that modulates the additional stabilizing or destabilizing effect of the non-singular stress acting parallel to the crack. Those properties predict the existence of an anisotropy threshold for crack kinking and explain the surprisingly strong dependence of this threshold on load distribution. The results provide the foundation to predict crack paths in diverse composites of practical interest.